1. Field of the Invention
The present invention relates to a process for removing carbon dioxide (CO.sub.2) and nitrogen oxides (NO.sub.x) from combustion gases. More particularly, it relates to a process for converting nitrogen monoxide (NO), the main constituent of NO.sub.x in combustion gases, into nitrogen dioxide (NO.sub.2) by oxidation with ozone and removing the NO.sub.2 together with CO.sub.2 by absorption from the combustion gases.
2. Description of the Related Art
It is common knowledge these days that the greenhouse effect of CO.sub.2 resulting from the burning of fossil fuels is responsible for the global warming and that other combustion products NO.sub.x cause photochemical smog and acid rain. Above all, acid rain has perniciously influenced natural ecosystems, directly attacking forests and farm products, indirectly affecting the plants through changes of soils, and acidifying lakes and rivers and thereby reducing the fish population. Since the air pollutants CO.sub.2 and NO.sub.x do extensive harm to the environment across international borders, reducing them is a problem of great urgency.
One approach already taken is recovery and removal of CO.sub.2 from combustion gases, e.g., coming out of boilers of thermal power plants that burn much fossil fuels, through contact with an aqueous solution of monoethanolamine (MEA), hindered amine, or the like.
On the other hand, the technology of wet denitration of combustion gases is difficult to develop, because the NO.sub.x concentration is low and more than 90% of NO.sub.x is accounted for by poorly reactive NO. As for dry denitration, a process is already in use which reduces NO.sub.x in combustion gas with the aid of a catalyst and with injection of ammonia. A typical wet process is oxidative absorption, which consists in scrubbing NOx-laden gas with water or a chemical solution, oxidizing NO to NO.sub.2 with an oxidizing agent or the like for effective removal of NO, and then scrubbing or directly washing with an oxidizing chemical solution.
The afore-described wet denitration process has drawbacks including high cost of the oxidizing agent to be used for favorable NO.sub.x absorption and the necessity of disposal of the used liquid absorbent. Combustion gases generally contain about 10% CO.sub.2, but their NO.sub.x contents are at most hundreds of parts per million, and the addition of an exclusive step for denitration would call for a large-scale equipment. As regards the simultaneous removal of CO.sub.2 and NO.sub.x, no satisfactory liquid absorbent has been available yet that is capable of removing both of them.
Thus, because of the influence of combustion gases over the environment, there is a strong desire for early development of a process that solves the problems of the prior art, is simple in operation, recovers both CO.sub.2 and NO.sub.x with high efficiency, and can be incorporated in existing facilities for the treatment of combustion gases.